Clay used in preparing polymer-clay nanocomposites has a layered-structure in which silicate plates are layered on a nanoscale by van der Waals' force. A polymer-clay nanocomposite makes the layered silicate structure exfoliated. This allows silicate to disperse uniformly on a nanoscale in polymer resins, so that the polymer resin can have better mechanical properties than conventional polymer resins and can achieve new properties such as gas shielding and thermal resistance that are not seen in conventional resins.
However, the clay used in a nanocomposite is itself hydrophilic and it is mainly acquired by treating natural montmorillonite with a metal cation. Further, there are difficulties in exfoliation and dispersion of the layered-structure into hydrophobic polymer resins because strong van der Waals' force acts between layers of silicate plates.
In order to solve those problems, U.S. Pat. Nos. 4,889,885 and 4,810,734 disclose a method of preparing an organic modified clay, in which a layered clay is subjected to ion exchange with an organic cation having a low molecular weight and whose structure has an affinity with polymer resin to expand the interlayer distance of the clay.
Because an organo-modified clay has micron sized particles, it is necessary to use a material which is capable of interaction between a polymer matrix and clay to disperse the clay on a nanoscale basis. In general, the clays may obtain compatibility with a polymer resin by conversion of the clays to ammonium or phosphonium form. However, conventional nanoclays in ammonium form have low thermal stability and are susceptible to decomposition during molding processes at around 200° C. or more, which results in poor elongation and toughness as well as discoloration in the nanocomposite. Therefore, conventional nanoclays in ammonium form have limits in application. On the other hand, nanoclays in phosphonium form have better thermal stability than nanoclays in ammonium form, but have low compatibility with polymer resins. Thus, nanoclays in phosphonium form show low improvement of flexural modulus and weak elongation.
Furthermore, organo-modified clays employing conventional organic modifiers have some disadvantages that they cannot be dispersed either in water or in any polar organic solvent.
Accordingly, the present inventors have developed a method of surface treating clay for polymer-clay nanocomposites by organic treatment of the surface of clay with a novel hyperbranched polymer having a three-dimensional structure, so that the clay can have good dispersibility in polar organic solvent and improved thermal stability.